Role of awns in ear water-use efficiency and grain weight in barley

Abstract - Two near-isogenic 6-rowed barley genotypes (Hordeum vulgare L cv Atlas), with and without awns, were grown in pots under Mediterranean conditions. The plants were watered daily. For each genotype, total net photosynthesis and water-use efficiency (WUE: net photosynthesis/transpiration) of ears and flag leaves were measured 3 times during grain filling: 1, 3 and 4 weeks after anthesis. The total photosynthesis of awned ears was markedly higher than that of awnless ears, until 3 weeks after anthesis. Thereafter, photosynthetic rates decreased faster in awned than in awnless ears. The WUE of awned ears was always higher than that of awnless ears. In fact, the WUE of awns alone was 2 to 3 times higher than the WUE of awnless ears during grain filling. The mature kernel dry weight was higher in the awned genotype than in the awnless genotype. Carbon isotope discrimination of kernels in the awned genotype was lower than in the awnless genotype. Thus, the presence of awns seem to help the ears to maintain higher rates of photosynthesis and WUE throughout the grain-filling period, and may be related to the greater kernel dry weight in the awned genotype, even under well-irrigated conditions. In an experiment in which either the ears or the flag leaves were darkened, the importance of the role of the ear was demonstrated, especially in the awned genotype, in supplying assimilates to the grain. When the ear was darkened, total grain yield per ear was reduced by 37.4% in the awned genotype, but by only 14.8% in the awnless genotype. Darkening the flag leaf increased carbon isotope discrimination in the mature kernels of the 2 genotypes, with respect to undarkened plants, especially in the awnless genotype. The alteration of the source-sink relationship in the plant may be responsible for this response to carbon isotope discrimination.